Field of the Invention
The invention relates to a technique of a safety valve and, in particular, to a safety valve that has a meltable body capable of being melt at a predetermined abnormal high temperature.
Description of Related Art
A meltable type safety valve is known as a safety device for preventing a pressure inside a pressure container from excessively rising when a temperature inside the container becomes an abnormal high temperature. The meltable type safety valve has a meltable body capable of being melt at an abnormal high temperature. At a normal temperature, displacement of a valve element is limited by means of the meltable body. At an abnormal high temperature, the meltable body is melt to release the limit of the valve element and displace the valve element, so that a flow path leading to the outside is opened and the pressure inside the container is reduced. A meltable type safety valve that has a meltable plug for use in the safety valve disclosed in Japanese Patent Application Publication No. 2012-132475 (JP 2012-132475 A), for example, exists as a meltable type safety valve having such a structure.
The meltable type safety valve disclosed in JP 2012-132475 A has a substantially cylindrical meltable body. In the meltable type safety valve, the fusibility of the meltable body is maintained and the strength of the meltable body is increased by means of a structure in which the strength of a part around an axis (a central part) of the meltable body is reduced and the strength of the circumference of the central part is increased. Moreover, in the meltable type safety valve in JP 2012-132475 A, damages of the meltable body in usual use (except in the case of an abnormal high temperature) are inhibited and wrong actions of the meltable type safety valve are prevented by increasing the strength of the meltable body.
In the meltable type safety valve disclosed in JP 2012-132475 A, a pressure inside the pressure container acts on the meltable body via the valve element, and damages of the meltable body caused by a creeping phenomenon are still worried about. In such a structure, in order to form a meltable type safety valve that can deal with a further high temperature and high pressure, the strength of the meltable body is required to be further increased. However, the strength of the meltable body itself is usually low, thereby it is hard to deal with the further high temperature and high pressure in the meltable type safety valve.